1. Field of the Invention
The present invention relates to a method for quantitatively detecting the decarburization reaction occurring in an annealing furnace during the production of an electrical steel sheet. More particularly, the present invention relates to a method for in situ detection of the decarburization reaction which occurs in an annealing furnace.
2. Description of the Related Art
An electrical steel sheet is used for the core of electrical machinery and devices. Electrical steel sheets are largely classified into two types, i.e., a non-oriented electrical steel sheet which is produced by utilizing primary recrystallization and normal grain growth and a grain-oriented electrical steel sheet which is produced by utilizing a phenomenon in which crystal grains having a particular crystal orientation, e.g., the orientation expressed by the Miller index as {110}&lt;001&gt;, are caused to grow abnormally, this abnormal growth being referred to as secondary recrystallization.
Carbon is one of the impurities of a non-oriented electrical steel sheet and is removed or decarburized during its production to as low a level as possible since the carbon remaining in the final product impairs the magnetic properties of the product and, with the lapse of time, the magnetic properties of the core in which the product is used gradually deteriorate. This deterioration is referred to as a magnetic aging phenomenon.
The carbon content of silicon steels can be decreased in the molten-steel processing stage including steelmaking and the pretreatment of molten steel. If it is attempted to attain in the molten-steel processing stage a low carbon content which is not detrimental to the magnetic properties, a sophisticated operation technique which is indispensable for attaining the desired carbon level renders the production efficiency at steelmaking very low. Therefore, some carbon usually remains in the molten-steel processing stage and is removed or decarburized at a later stage. This decarburization is usually carried out during the annealing stage, i.e., decaburization is attained together with primary recrystallization and normal grain growth during annealing.
In the production of a grain-oriented electrical steel sheet, a certain carbon content is necessary for realizing stable secondary recrystallization. Molten steel has, therefore, a carbon content of from approximately 0.02% to 0.06% so that such carbon content of a strip creates a condition for stable secondary recrystallization.
The necessity of controlling the carbon content in the range above has long been known in the pertinent technical field.
Japanese Unexamined Patent Publication No. 58-55530 discloses a representative method for controlling the carbon content. In the method disclosed in this publication, the product is obtained by the following processes: single pass cold-rolling of a hot-rolled strip or a multi pass of cold-rollings with intermediate annealing to obtain the final thickness of the product; decarburizing annealing in a wet hydrogen atmosphere; applying and then drying powder mainly composed of MgO; and subsequently annealing at a temperature exceeding 1100.degree. C. The aims of this annealing carried out at the final production step are to generate secondary recrystallization and to form a ceramic insulating material mainly composed of 2MgO.SiO.sub.2 due to the reaction between SiO.sub.2, which is formed on the sheet surface during the decarburizing-annealing step, and MgO, which is applied on the sheet surface. An aim of decarburizing-annealing is to reduce the carbon content of the steel sheet prior to secondary recrystallization annealing to as low as possible, usually 0.002% or less, thereby stably generating secondary recrystallization.
Conventionally, the decarburization of a hot-rolled steel sheet is carried out only in the decarburizing-annealing step. As is disclosed in Japanese Unexamined Patent Publication No. 58-5530 mentioned above, the optimum carbon amount for secondary recrystallization depends on the carbon amount in the cold-rolling step. Therefore, according to a recently employed decarburization method, decarburization is carried out in the annealing step of a hot-rolled strip or in the intermediate annealing step so as to provide such a complete decarburization as previously attained in the decarburizing annealing step.
In summary, no matter what kind of non-oriented or grain-oriented electrical steel sheet is to be produced decarburization is carried out by annealing.
Incidentally, decarburization proceeds, during the decarburizing-annealing step, according to the following reactions: C(in Fe-Si)+H.sub.2 O.revreaction.CO+H.sub.2. Since the amount of carbon in steel is great at the initial period of annealing, the amount of CO formed is great. If a considerable amount of CO is formed, the reaction mentioned above proceeds in the left direction or an oxide is formed on the surface of the steel sheet, with the result that the decarburization rate is retarded. In this case, it is necessary either to increase the H.sub.2 O or to dilute the formed CO with a supplying gas medium in order to promote decarburization. In an annealing period where decarburization proceeds and the carbon amount in steel is thus decreased, the amount of formed CO becomes low. In this case, unless the H.sub.2 O amount in the gas annealing atmosphere is decreased, an oxide of high order is formed on the surface of the steel sheet. It is accordingly necessary to detect the H.sub.2 O and CO amounts in the annealing atmosphere and to control the annealing atmosphere depending upon the detected H.sub.2 O and CO amounts. This control is effected by supplying gases into the annealing furnace or adjusting the dew point of the gas atmosphere of the annealing furnace. Such control is the strictest in the decarburizing-annealing step, which is carried out after the final cold-rolling step of production of a grain-oriented electrical steel sheet, as is described in detail hereinafter.
The principal aims of decarburizing-annealing are to cause primary recrystallization of the cold-rolled steel sheet and decarburization, as well as to form a silica-scale layer, i.e., an oxide film. The silica-scale layer formed in the decarburizing-annealing step exerts great influences on the formation of a primary film, i.e., a forstellite film which is formed at a later step than the decarburizing-annealing step. The formation of a silica-scale layer and its properties therefore play an important role in determining the power loss characteristics of the articles in which the grain-oriented electrical steel sheet is used.
The H.sub.2 O, which participates in the above-described decarburization reaction C+H.sub.2 O.revreaction.CO+H.sub.2 and which is brought into reaction with the carbon in steel, is the water vapor contained in the gas atmosphere of the annealing furnace. In the decarburizing-annealing step, decarburization by H.sub.2 O first occurs. If, however, the H.sub.2 O partial pressure is too high, a film of oxides, such as FeO, Fe.sub.2 O.sub.3, and the like, is formed on the surface of the steel sheet and impedes the contact between the H.sub.2 O and C, thereby suppressing decarburization and hence impairing the magnetic properties.
The H.sub.2 O then reacts with the Si contained in the steel in a later half period of the decarburizing-annealing step and causes the formation of a silica-layer scale, i.e., a film of oxides, such as SiO.sub.2, 2FeO.SiO.sub.2, and the like, according to the following reactions: EQU Si+2H.sub.2 O.fwdarw.SiO.sub.2 +2H.sub.2 EQU 2Fe+Si+4H.sub.2 O.fwdarw.2FeO.SiO.sub.2 +4H.sub.2
If these reactions proceed to the extent of excessive oxidation, the adhesiveness of the film is impaired and the film thickness exceeds that of requisite primary film, resulting in a reduction in the space factor of a core and impairment of the film properties. Oxidation by the above reactions should be controlled to provide the oxide film with an amount and composition appropriate for forming a primary coating having an excellent quality since the amount and composition of the oxide film exerts an influence on the quality of glassy insulating film mainly composed of 2MgO.SiO.sub.2 and referred to as the primary coating.
As is described above, both the decarburization reaction and the oxidation reaction for forming an oxide layer occur in the decarburizing-annealing step.
To attain compatible satisfactory decarburization and appropriate formation of the oxide layer, it is important that the gas atmosphere, dew point, time, and the like of decarburizing-annealing be delicately controlled.
Conventionally, the partial-pressure ratio P.sub.H.sbsb.2.sub.O /P.sub.H.sbsb.2 of the water vapor and hydrogen gas or the dew point of the gases of the annealing furnace is controlled, such as is disclosed in Japanese Examined Patent Publication No. 58-43,691. The partial-pressure ratio and the dew point are determined by measuring the proportion of water vapor to hydrogen prior to admitting the supply gases into the annealing furnace. Alternatively, as is described in "Steel Handbook IV, 3rd Edition", edited by the Japan Institute for Iron and Steel, page 561, a dew probe, in which the hygroscopic saturation characteristic of lithium chloride is employed, is attached to the annealing furnace, and the gas atmosphere of the furnace is sucked outside the furnace into the dew probe to measure the dew point. In this case, the gas atmosphere at a portion of the furnace interior is used to measure the dew point. The so-detected values are not considered to provide true information on decarburization and oxide-film formation since the partial-pressure ratio and dew point undoubtedly greatly vary spatially within the huge space of the annealing furnace.
By "true information", the inventor means the state of the gas atmosphere in the neighborhood of the steel sheet. The inventor understands the necessity of quantitatively recognizing decarburization including the formation of an oxide film and occurring due to reactions between the surface of the steel sheet and water vapor present in the vicinity of the surface.
Since conventional methods for measuring the dew point and the like involve points to be improved, conventional decarburization-controlling methods cannot be said to be satisfactory. That is, since an appropriate method for obtaining true information has not heretofore been provided, decarburization control not based on true information has been carried out previously.